Anode

ABSTRACT

THE INVENTION IS AN ANODE SUITABLE FOR USE IN AN ELECTROLYTIC CELL. IT COMPRISES A BOX-LIKE STRUCTURE, OPEN AT EITHER END, OF TITANIUM OR A SIMILAR METAL AND HAS AT LEAST TWO PARALLEL PLATES HAVING AT LEAST PART OF THEIR OUTER SURFACES COATED WITH A PLATINUM GROUP METAL. MEANS ARE PROVIDED TO PREVENT SUBSTANTIAL DEFORMATION OF THE ANODE WHEN THE LOWER EDGE IS CAST IN A CONDUCTING METAL IN THE CELL.

D. C. M LEAN May 9., 1972 ANODH l3 Sheets-Shwt 1 Filed Oct. 26, 1967 FIG./

y 1972 D. c. MCLEAN 3,661,757

ANODE Filed Oct. 26, 1967 2 Sheets-Sheet 2 FIG. 2

United States Patent 3,661,757 ANODE Derek Colin McLean, Sandbach, England, assignor to Murgatroyds Salt and Chemical Company Limited, Sandbach, England Filed Oct. 26, 1967, Ser. No. 678,343 Claims priority, application Great Britain, Nov. 8, 1966, 49,880/ 66 Int. Cl. B01k 3/04 US. Cl. 204-290 F 3 Claims ABSTRACT OF THE DISCLOSURE The invention is an anode suitable for use in an electrolytic cell. -It comprises a box-like structure, open at either end, of titanium or a similar metal and has at least two parallel plates having at least part of their outer surfaces coated with a platinum group metal. Means are provided to prevent substantial deformation of the anode when the lower edge is cast in a conducting metal in the cell.

The invention relates to electrolytic anodes for use in electrolytic diaphragm cells for the electrolysis of aqueous solutions of alkali metal halides.

Electrolytic diaphragm cells for use in the production of gaseous chlorine and hydrogen together with caustic alkali by the electrolysis of aqueous solutions of alkali metal halides, in particular sodium chloride, are well known. Certain types of cells e.g. those commonly known as Hooker cells, have employed a pluarlity of foraminous or perforated metal cathodes e.g. fabricated of steel wire mesh, covered with a liquid-permeable diaphragm or membrane interleaved within the cell with a plurality of anodes which in general are parallel, vertically disposed fiat blades of graphite having their lower ends cast in a slab of a fusible metal such as lead forming part of the base of the cell. It is usual also to cast one or more copper conductor bars in the lead slab in order to conduct the electrolysing current from an external source to the anode blades. Alternatively, as is described in British patent specification No. 880,838, the anodes may be mounted in slots formed in one or more copper grids, which serve to conduct the current from an external source, electrical connection being made between the graphite anodes and the grid or grids by mounting the assembly in a shallow tray and pouring in sufficient fusible metal such as lead to cover the grids. In order to protect the lead and copper conductor from the corrosive attack of the chlorinated brine during the operation life of the cell a protective layer of a material such as concrete or bitumen is applied over the lead slab. In operation of such cells it has been found that the graphite anodes are attacked and wear progressively thinner during use. Such a thinning of the anodes leads to a progressive increase in cell voltage if the current is held constant because the cross-section of graphite that is carrying the electrolysing current is reduced, and the current path through the electrolyte from anode to cathode is progressively increased.

Moreover, during elecrolysis using graphite anodes the surface disintegration of the graphite leads to contamination of the electrolyte and the plugging of the cathode diaphragm or membrane by oily and carbonaceous material which impairs the efiicient operation of the diaphragm and shortens its life. Also carbon dioxide is formed by attack on the anodes and this contaminates the gaseous chlorme.

It is an object of the present invention to provide an anode for use in an electrolytic diaphragm cell which substantially avoids the above disadvantages.

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Accordingly the present invention is an anode, suitable for use in a diaphragm cell for the electrolysis of aqueous solutions of alkali metal halides, wherein the cell contains a plurality of parallel, vertically disposed anodes having their lower ends cast in a conducting metal, the said anode comprising a box-like structure, open at each end, of titanium or similar metal with at least two parallel plates having at least part of their outer facing surfaces plated with a platinum group metal, with means at the lower edge to substantially prevent deformation of the structure on casting in the conducting metal base, the structure being such as to allow open access to the interior of the anode.

Throughout this specification the expression titanium or simlar metal is defined as meaning titanium or titanium-base alloys or niobium or tantalum or alloys of titanium, niobium, or tantalum which when made anodic Within a diaphragm cell of the type described herein forms an insulating film having an electrical resistance comparable to or greater than that formed on titanium under identical conditions.

By the term platinum group metal herein is meant any of the metals platinum, palladium, rhodium, ruthenium, osmium or iridium or alloys of any two or more such metals. The preferred metals are platinum, palladium or iridium.

The box-like structure of the anode may be constructed in any convenient manner, for example by foldingg from a single sheet of titanium or similar metal to provide the two parallel plates, the folded sheet being suitably welded e.g. spot welded, along the edges opposite the fold to present a rigid unit. Alternatively the anode may consist of two parts which can be joined together by spot welding or other suitable means. The structure may be strengthened by ribbing of the parallel plates or by bending the parallel plates at the edges and/or by the provision of internal spacers, for example members of channel section, in the structure.

'Means are required to prevent deformation of the anode structure arising from the uneven temperature distribution that is set up in the anode material during casting or soldering in the metal base. For instance, if the lower portion of the anode, initially at atmospheric temperature, is immersed in molten lead at say 350 C., the heated portron will expand by more than & in every 12". Since titanium is not a good conductor of heat, compared to other metals such as copper, a temperature gradient will be set up between the bottom and the top of the anode which will tend to distort the thin sheet material of which the anode is constructed. Also, after the conducting metal in which the anode is cast has solidified, it will continue to contract until it reaches room temperature. This contraction is imposed on the lower portion of the anode. The main portion of the anode will consequently be put mto compression in a transverse direction and will tend to buckle. If distortion is allowed to take place, accurate parallel alignment of the anode and cathode elements will not be achieved. The means substantially to prevent deformation of the anode structure on casting in the metal base may take any suitable form. For example a horizontal spacer may be Welded between the two parallel plates at a level just above the level of the surface of the conducting metal into which the anodes are cast, but not substantially above the level of the lower limit of the cathode elements in the assembled cell. Further horizontal spacers may also be fitted at higher levels. Alternatively or in addition, the lower edges of the parallel plates may be crimped or provided with a series of slots which tend to remove or even out the strain produced in the structure during casting in the conducting metal. The depth of the crimping or slots above the lower edge of the anode structure should be suflicient to extend above the surface of the conducting metal into which the anode is cast. The precise dimensions of .such slots or crimping will depend upon the size of the anode itself and the depth to which the anode is immersed in the cast metal. Thus, for example with an anode having parallel plates measuring about 18 inches square and a depth of casting about 1 inch, a depth of crimping or slot of up to about 5 inches above the metal surface of the cast base may be found sufficient to substantially prevent deformation while the casting metal is being poured. Moreover, when slots are used they should be of such width as to allow the passage of molten metal during casting in the conducting metal and also for the passage of protective coating e.g. bitumen or concrete, subsequently applied to the base. However, such slots should not be too wide since this leads to an undesirable increase in current density per unit area. This would increase the voltage drop when the electrolysing current is conducted through the anode material and will consequently increase the operating voltage of the cell. With an anode of the dimensions quoted above a slot width of about A" to about A" may be suitable. The lower edges of the anode structure may also be provided with holes so as to key the anode into the conducting metal. The cast metal is suitably lead or a lead alloy.

In a preferred embodiment, the lower edges of the parallel plates are provided with a series of slots, and a bar of rigid material having a low coefiicient of thermal expansion is bolted to the ends of the slotted section. Materials that may be suitable include baked carbon and graphite. This bar resists the contraction of the conducting metal and largely eliminates distortion of the anode during the period of cooling after the cast metal bar solidiified. To prevent relative movement of the titanium and the graphite it is possible either to use clearance holes and friction grip bolts or fitted holes with precision bolts or pins. As a further variation it may be desirable to use precision bolts or pins with clearance holes allowing limited movement. This may be desirable for the following reason; the coefficient of expansion of graphite is so low that it virtually prevents any relative movement of the sections of the anode that are embedded in the conducting metal, for example lead, during the casting and cooling of the lead. However, when the anode is put to work the temperature of the whole assembly is raised to 90-95 C. The main portion of the anode will therefore expand while the lower edge is prevented from expanding by the graphite. If clearance holes are used in the graphite the clearance can be so calculated that while the outer sections at the lower edge of the anode will have been nipped in after cooling to room temperature, and some strain produced, this strain is exactly relieved on heating to operating temperature by virtue of the high coefficient of thermal expansion of lead compared to titanium.

For example, if the lower edge of the titanium and the graphite bar are drilled with the holes in precise register, and if the holes in the titanium are drilled to precise size while the holes in the graphite are drilled with a clearance with respect to the bolts of a mm. on diameter, then the permitted differential expansion will be a mum. in either direction.

Consider 2 bolts with their centres spaced 100 mm. apart.

If the assembly has been cooled to 25 C. after casting the lead and the operating temperature is 95 C., the upper part of the titanium anode, which is immersed in the anolyte, will expand 70 x 8.9 x lO X 100 mm.= 0.062 mm. in every 100 mm. (Mean coefficient of linear thermal expansion of titanium 8.9x 10* mm./ C.) The lead in which the lower edge of the anode is cast will, however, expand 70 x 29.4 x 10 x 100:0.21 mm. in every 100 mm. over the same temperature range. (Mean coeflicient of linear thermal expansion of graphite 29.4 10- mm./ C.)

The lead will therefore expand 0.15 mm. more than the titanium. The titanium will not be subject to strain at the operating temperature if the lower edge of the anode is nipped in to this extent during the contraction of the lead after casting. This object will be achieved if, for example, fitted holes are drilled in the titanium and clearance holes in the graphite, allowing a clearance of 0.15 mm. on diameter for every mm. pitch between bolt centres.

Good electrical contact between the lower part of the anode an dthe cast metal is preferably ensured by tinning or platinising the lower sections of the anode structure or alternatively by intensive cleaning of such sections by for instance sand or shot blasting or by etching.

The parallel plates of titanium or similar metal have at least part of their outer facing surfaces, that is those surfaces which in the assembled cell are directly opposite to a cathode surface, plated with a platinum group metal and preferably with platinum metal itself. A suitable method of effecting such a plating is described and claimed in British specification 885,819.

The anode of the present invention is also preferably provided with means to enable lifting of the assembly of anodes when cast in the conducting metal. Such holding means may be a hole or holes in the side members of the anode structure. These holes serve to provide a means for holding the anodes in the necessary alignment to a jig prior to and during casting in the conducting metal. Thereafter the jig is still attached to the anode assembly cast metal and copper conducting bar or bars may be transported by an operhead crane or hoist to the site for the assembly of the cell.

It is also an essential feature of the diaphragm cell of the present invention that the anode structure be such as to allow open access to the interior of the anode. This is necessary to allow for the treatment of the surface of the metal base within the box-like structure of the anode with one or more protective layers of say bitumen or concrete. Moreover, it is necessary to allow for ease of circulation of the electrolyte through the cell during operation. This open access may be suitably provided by cutting away the side members of the anode structure to a suitable level above the surface of the uppermost protective layer.

The cathode and general cell construction are well known and conventional in the art. The cell base may be fabricated of concrete, suitably protected mild steel or suitably protected cast iron.

Anodes of the present invention are described in more detail with reference to the accompanying drawings which show perspective part views.

Referring to FIG. 1, a titanium or similar metal sheet 1 is folded vertically to provide two vertically disposed parallel plates 2 and 3. Two overlapping portions of the metal sheet opposite the fold are spot welded at 4 to present a rigid unit. This form of construction is particularly preferred since it avoids the use of fusion welding which is difficult and costly with titanium or similar metal. The outer facing surfaces of the parallel plates are plated with platinum metal in the area indicated generally as P. The outer facing edges may also be platinised. The lower adjacent corners and part of the side members of the box-like structure of the anode are cut away at 5 to allow open access into the interior of the anode structure. The lower edges of the parallel plates are provided with slots 6 which act substantially to prevent deformation of the structure on casting in the conducting metal (not shown). The lower edge sections of the parallel plates are also plated with platinum metal or tinned in the areas marked T to provide for better electrical contact between the anode metal and the conducting metal in the assembled cell. Holes 7 may be prow'ded in the upper sections of the sidemembers of the anode structure and these act as holding means for attaching a jig to the anode to enable lifting of the assembly of anodes when cast in the conducting metal.

FIG. 2 shows two similar sheets of titanium or similar metal which provide two vertically disposed parallel plates 2 and 3 which are spot welded together. The lower adjacent corners and part of the side members are again cut away at 5 to allow open access to the interior of the anode. The lower edges are provided with slots 6 substantially to prevent deformation on casting of the conducting metal. Holes 7 are provided to act as holding means for attaching a jig to the anode. A graphite bar 8 is bolted to the ends of the slotted section by means of bolts 9. Two channel sectioned members 10 and 11 are provided as internal spacers to strengthen the structure.

I claim:

1. An anode, suitable for use in a diaphragm cell for the electrolysis of aqueous solutions of alkali metal halides wherein the cell contains a plurality of parallel, vertically disposed anodes having their lower ends cast in a conducting metal, the said anode comprising a box-like structure open at each end, of titanium or similar metal with at least two parallel plates having at least part of their outer facing surfaces plated with a platinum group metal, the lower edges of the parallel plates being provided with a series of slots and a bar of rigid material having a low coefiicient of thermal expansion being bolted to the ends of slotted section, the structure being such as to allow open access to the interior of the anode.

2. An anode according to claim 1 wherein the rigid material having a low coefiicient of thermal expansion is baked carbon or graphite.

3. An anode according to claim 2 wherein the graphite is held with clearance holes in precision bolts or pins allowing limited movement.

References Cited UNITED STATES PATENTS 1,376,495 5/ 1921 Williams 204-266 1,513,728 11/1924- Allan 204--284 3,085,967 4/1963 Motock a 204--284 3,123,545 4/1964 Williams 204-284 3,412,000 11/1968 Bedi 204--290 F 3,425,929 2/ 1967 Emery et a1. 204-266 WINSTON A. DOUGLAS, Primary Examiner H. FEELEY, Assistant Examiner 

